1. Field of the Invention
The present invention relates to an exhaust gas purification device for an internal combustion engine having a particulate filter for trapping exhaust particulates discharged from the engine. In particular, the present invention relates to deposit amount calculation of each component of exhaust particulates and regeneration treatment of a particulate filter of an exhaust gas purification device for an internal combustion engine.
2. Description of Related Art
For environmental protection, an exhaust gas purification device is introduced to reduce air pollutants discharged from an exhaust port of an internal combustion engine. As a conventional exhaust gas purification device, a catalyst system such as an oxidation catalyst, a NOx catalyst or a three-way catalyst is publicly known. Further, elimination of exhaust particulates contained in exhaust gas of a diesel engine is required. For the elimination of the particulates, an exhaust gas purification device having a particulate filter in an exhaust passage is put into practice. The particulate filter traps the exhaust particulates in the exhaust gas by forcing the exhaust gas to pass through porous partition walls. The trapped exhaust particulates deposit on surfaces or in fine bores of the partition walls.
If a particulate deposit amount becomes excessive, a flow resistance in the particulate filter is increased and a back pressure of the internal combustion engine is increased. As a result, an engine output is reduced. In order to avoid this problem, regeneration treatment of the particulate filter (regeneration treatment for eliminating the particulates) is executed on a regular basis to burn and eliminate the deposited exhaust particulates. In the regeneration treatment of the particulate filter, it is required to accurately estimate the particulate deposit amount and to execute the regeneration treatment at appropriate timing.
JP-A-H07-332065 describes that, on a basis that a pressure loss in the particulate filter increases due to deposition of the exhaust particulates, a particulate deposit amount is obtained from a pressure difference across the particulate filter. Since the pressure difference depends on an exhaust gas flow amount, it is preferable to calculate the particulate deposit amount based upon the pressure difference and the exhaust gas flow amount. The regeneration treatment is executed if it is determined that the regeneration treatment is necessary when the particulate deposit amount exceeds a predetermined value.
JP-A-2001-280118 describes an apparatus that has an oxidation catalyst upstream of a particulate filter and treats trapped exhaust particulates by nitrogen dioxide (NO2) generated by oxidizing nitrogen monoxide (NO) in the exhaust gas. Thus, the apparatus continually regenerates the particulate filter. This apparatus calculates a particulate discharge amount from engine operation information and calculates a particulate combustion amount from particulate filter temperature information. The apparatus integrates a difference between the calculated particulate discharge amount and the calculated particulate combustion amount to determine a particulate deposit amount in the particulate filter. The apparatus calculates the particulate discharge amount and the particulate combustion amount by using a particulate discharge amount map and a particulate combustion amount map stored in a controller beforehand.
Generally, particulates deposited in the particulate filter are comprised of solid soot (carbon) and a liquid organic solvent soluble component (soluble organic fraction (SOF): hydrocarbon) adhering to the periphery of the soot. The soot has high combustion temperature (for example, 600° C. or higher) and does not cause spontaneous combustion frequently during operation. However, the SOF burns at relatively low temperature (for example, 250° C. or higher). Therefore, in the case where a large amount of the SOF is deposited, there is a possibility that heat generated in the combustion of the SOF triggers rapid combustion of the soot even if exhaust gas temperature is low. As a result, a filter substrate or a catalyst is damaged if the temperature of the particulate filter is increased to high temperature over allowable limit temperature.
Exhaust gas temperature of 250° C., which causes the combustion of the SOF, frequently occurs during normal running of a vehicle. Accordingly, it is required to prevent abnormal combustion of the soot due to the SOF combustion in order to use the particulate filter safely. JP-A-H07-332065 uses the pressure difference across the particulate filter to obtain the particulate deposit amount. The pressure difference is strongly associated with the soot deposit amount. However, the pressure difference is not strongly associated with the SOF amount. Therefore, it is difficult to estimate the SOF deposit amount based upon the pressure difference. JP-A-2001-280118 obtains the amount of particulate deposits containing the SOF and the soot without discriminating the SOF and the soot. Accordingly, if the regeneration treatment is executed in an early stage in order to avoid the above-described problem, fuel consumption deteriorates due to an increase of frequency of the regeneration treatment. As described above, according to the conventional art, it is difficult to grasp the SOF deposit amount and to carry out the regeneration treatment at appropriate timing in such a manner as not to induce rapid combustion of the soot.